Strange alien world made of 'hot ice'

A bizarre world of scorching hot ice shrouded in a steamy atmosphere may have been found, according to new observations. Characterising the Neptune-size planet is an important milestone on the way to detecting and characterising Earth-like planets that could harbour life.

Astronomers have discovered more than 200 planets orbiting other stars, called extrasolar planets or exoplanets. Almost all of these were detected by the way their gravity makes their parent stars wobble. But this technique, called the radial velocity method, reveals very little about the planet except for the size of its orbit and an estimate of its mass.

Astronomers can learn a lot more by watching "transits" of planets that pass in front of their parent stars as seen from Earth. Careful analysis of the dimming this causes can provide clues to the planet's composition and structure. But the brightness dips are small and difficult to detect for all but the largest planets.

Now, astronomers have observed the smallest ever transiting planet. It has turned out to be a strange world, unlike anything seen before.

The planet, which orbits a small star located 30 light years from Earth called GJ 436, was actually discovered in 2004 using the radial velocity method (see Two new rocky super Earths found). At that point, astronomers deduced that it was about as massive as Neptune.

Exotic ice

But now, a team led by Michael Gillon of Geneva University in Switzerland have observed the planet transiting its host star using a telescope at the Observatoire Francois-Xavier Bagnoud (OFXB) in Saint-Luc, Switzerland.

They have been able to measure the planet's width, which provides clues to its composition and structure. It turns out to be about 50,000 kilometres wide, roughly four times the width of Earth and about the size of Neptune.

The planet is therefore too compact to be made mostly of hydrogen gas, like Jupiter, the researchers say, but not compact enough to be a rocky 'super Earth', as some had speculated. Instead, they believe it must be made mostly of an exotic form of water.

Although the parent star is much cooler than the Sun, the planet orbits 13 times closer to the star than Mercury's orbit around the Sun. That means the surface must be a blazing hot 300° C or more, keeping water in its atmosphere in vapour form.

But the high pressures in the planet's interior would compress the water so much that it would stay solid even at hundreds of degrees Celsius - the expected temperatures inside the planet. There are a variety of exotic 'hot ice' states possible in such conditions, with names like 'Ice VII' and 'Ice X'.

Ocean worlds

"Water has more than a dozen solid states, only one of which is our familiar ice," says team member Frederic Pont of Geneva University. "Under very high pressure, water turns into other solid states denser than both ice and liquid water, just as carbon transforms into diamond under extreme pressures."

The inferred composition of the planet is very much like that of Neptune, which is also made mostly of ice, Pont says. "If you bring Neptune nearer to the Sun and it's heated outside to 300° C, that's exactly what you get," he told New Scientist.

Water would not have condensed to form the GJ 436 planet so close to its star, so it must have formed farther out and migrated inwards, he says. Other similar planets out there could have stabilised at the right distance from the star to become "ocean planets", he says.

Since astronomers have only searched a tiny fraction of stars in our galaxy, finding one planet like this suggests there are many more out there, including some ocean worlds, he says. "To me, it proves that there are many planets with liquid water, because if there's one like this, it could have been a bit further from the star and then the temperature would have been right," he says.

'New era'

Sara Seager, an exoplanet expert at the Massachusetts Institute of Technology in Cambridge, US, says the finding is "tremendously exciting" because it is the first time anyone has been able to detect a transit of a planet smaller than a gas giant.

"It's heralding a new era in exoplanets," she told New Scientist. "It's opening a door to identifying habitable planets in the near future."

But she says the composition and structure of the planet are still uncertain. Although it could be mostly ice, as Gillon's team suggests, it is possible to imagine other compositions that would fit the data, she says, such as a rocky world with a massive atmosphere.

Even if water is abundant in this planet, the conditions would not be right for life to exist there, she says. It would be too hot, and it is not even clear if any of the water could be in liquid form, although deep inside where the pressure goes up, there could be a region where the water is in a quasi-liquid state. "It could pass through a strange region where it's not quite solid and not quite liquid," she says.

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Transits occur when a planet appears to pass in front of its star as seen from Earth. New observations reveal that the planet around the star GJ 436 is just four times as wide as Earth - the smallest transiting planet yet known (Illustration: NASA/ESA/G Bacon)